1. Field of the Invention
The present invention relates generally to an automatic transmission primarily intended for motor vehicle use, and more particularly, to a method of invoking a shutdown on default of an electronic controller of an automatic transmission.
2. Description of Related Art
Automatic transmissions are designed to take automatic control of frictional units, gear ratio selection and gear shifting. In general, the major components featured in such an automatic transmission are: a torque converter; fluid pressure-operated multi-plate drive or brake clutches and/or brake bands which are connected to the individual elements of planetary gear sets in order to perform gear shifts; and transmission control such as valves for applying and releasing elements to shift the gears.
The control system of the automatic transmission is typically hydraulically operated through the use of several valves to direct and regulate the supply pressure. However, in recent years, a more advanced form of transmission control system has been proposed, which would offer the possibility of enabling the transmission to adapt itself to changing conditions. In this regard, U.S. Pat. No. 4,875,391, issued on Oct. 24, 1989, to Leising, et al., sets forth the fundamental development in this field. Specifically, this patent discloses an automatic transmission which features an "adaptive" control system that includes electrically operated solenoid-actuated valves which are controlled by an electronic controller. The controller shifts the transmission by applying a predetermined combination of valves which are under software control.
During operation of the transmission, the controller software monitors the transmission for proper operation by testing for: input and output speeds/ratios and engine speed; pressure checks/tests, e.g., presence of pressure only at the clutches that the controller has applied; and data from sensors and other peripherals. If a fault is detected by any of these tests for a sufficient length of time, the controller software will turn off a shutdown relay. By doing so, electrical power is cut off to the solenoid-actuated valves, thereby rendering the transmission in a "default" or "limp-in" mode.
One disadvantage of the above is that once the shutdown relay is turned off, the relay cannot be turned on unless the electronics of the controller are reset. The reset may be accomplished by turning the vehicle's ignition off and then back on. Until the reset is performed, the transmission is held in the limp-in mode indefinitely. This indefinite "default" is undesirable because the detected faulty operation may only be temporary and, regardless, may put the transmission in an indefinite default state; the operator of the vehicle may not know that the default occurred and may drive in the default mode for long periods in spite of a temporary fault; and/or unnecessary unrecoverable default operation of the transmission may compromise vehicle driveability, durability and fuel economy.